In hysteretic control a sense parameter, e.g., voltage, current, temperature, pressure, relative humidity etc., cycles between first and second thresholds, e.g., upper/lower and lower/upper thresholds. There is no error amplifier or complex pole-zero filters used or needed. Hysteretic control is sometimes called “Bang-Bang” control where the system alternates between two states, e.g., on or off, high or low, hot or cold, fast or slow, up or down, right or left, clockwise or counterclockwise, etc. Hysteretic control turns on/off a controlled device when the sense parameter is less than the first threshold and turns off/on the controlled device when the sense parameter is greater than the second threshold, e.g., thermostat and heating/cooling system. Switching frequency may vary with changing loads and operating conditions, e.g., temperature.
Simple hysteretic control may use a single hysteretic analog comparator. Advanced asymmetric hysteretic control methods may support adaptive behavior, e.g., adjusting for changing loads, maintaining efficiency over changing conditions, etc. Asymmetric hysteretic control requires two references, e.g., digital-to-analog converters (DACs) and two comparators for independent first and second thresholds. Timers may also be required to provide for frequency control. These components result in significantly more expensive hysteretic control.